The present invention relates to glass for use in cathode ray tubes, and more particularly to glass for use in monochromatic colored light emitting, or light-source, cathode ray tubes for using to display a colored picture image on a huge screen installed in a score board of a stadium or in an outdoor ad pillar, which has been developed recently.
In the huge screen color imaging system, a set of three light-source cathode ray tubes form one picture element, on the inner surfaces of the respective glass bulbs of which are deposited red-, green- and blue-phosphor materials, respectively, to form cathode ray tubes for monochromatically emitting red, green and blue lights, respectively, as excited by electron beams from electron guns mounted within the respective glass tubes. A plurality of sets of three light-source cathode ray tubes are arranged in a matrix form to form the huge screen.
Such color imaging systems are necessitated to be of all-weather type and capable of displaying a clear picture image even in broad daylight. Accordingly, a light-source cathode ray tube is desired to have a high brightness and a good contrast while suppressing reflection of an external light. To that end, it is desired that a glass bulb of a light-source cathode ray tube should be highly transparent for the light emitted from the phosphor material and having the wavelength of the red, green or blue color, but should fully absorb the color light of the other wavelengths, for example, the yellow light such as the sun light at dusk, to which the human eyes are most sensitive as accompanied by dazzling, to clearly reproduce the respective red, green and blue colors.
In conventional color television tubes, it is known as disclosed in British Pat. No. 1,154,500 to apply an optical filter glass to a face plate, or a safety glass, of the color television tube, which has selective absorption and transmittance properties so that the transmission of light in the red, the green and the blue portions of the spectrum is permitted at a high rate but the transmission of the yellow portion is reduced, in order to provide improved color, contrast, and viewing characteristics.
Since the conventional color television tube displays a colored picture on its screen, such selective absorption and transmittance properties permitting sufficient transmission of the red, the green and the blue portion of the spectrum is important. But the glass is not useful for the light-source cathode ray tube, because the one colored light, or the red light emitted from the phosphor of a cathode ray tube is disturbed from the other portions of the spectrum, or the green and the blue portion of the external light reflection, so that the contrast is degraded.
Accordingly, in order to improve contrast of light-source cathode ray tubes, it will be first conceived to use respective glass bulbs individually colored red, green and blue, and it will be thought of that cadmium-selenium colored glass is used as the red glass, chromium colored glass is used as the green glass and cobalt colored glass is used as the blue glass. However, among these kinds of colored glass, while the manufacture of the cobalt colored blue glass and the chromium colored green glass is relatively easy, with regard to the cadmium-selenium colored red glass, especially the cadmium content comes into problem in view of a pollution and hence its manufacture is difficult. In addition, it is not economical to respectively manufacture such three kinds of colored glass.
In addition, in a cathode ray tube, since a high operating voltage is applied and thereby X-rays and electron-rays are generated, it is necessary that its glass bulb should have the capability of absorbing X-rays and should not be subjected to browning and discoloration which results in lowering of a transmittance coefficient due to X-ray and electron irradiation over a long period, but this filter glass does not have such characteristics.